diff --git a/src/type/definition.d.ts b/src/type/definition.d.ts
index b37ea16dad..457e756dd4 100644
--- a/src/type/definition.d.ts
+++ b/src/type/definition.d.ts
@@ -171,21 +171,16 @@ export function assertAbstractType(type: any): GraphQLAbstractType;
  *     })
  *
  */
-interface GraphQLList<T extends GraphQLType> {
+export class GraphQLList<T extends GraphQLType> {
   readonly ofType: T;
+
+  constructor(type: T);
+
   toString: () => string;
   toJSON: () => string;
   inspect: () => string;
 }
 
-interface _GraphQLList<T extends GraphQLType> {
-  (type: T): GraphQLList<T>;
-  new (type: T): GraphQLList<T>;
-}
-
-// eslint-disable-next-line @typescript-eslint/no-redeclare
-export const GraphQLList: _GraphQLList<GraphQLType>;
-
 /**
  * Non-Null Modifier
  *
@@ -206,21 +201,16 @@ export const GraphQLList: _GraphQLList<GraphQLType>;
  *
  * Note: the enforcement of non-nullability occurs within the executor.
  */
-interface GraphQLNonNull<T extends GraphQLNullableType> {
+export class GraphQLNonNull<T extends GraphQLNullableType> {
   readonly ofType: T;
+
+  constructor(type: T);
+
   toString: () => string;
   toJSON: () => string;
   inspect: () => string;
 }
 
-interface _GraphQLNonNull<T extends GraphQLNullableType> {
-  (type: T): GraphQLNonNull<T>;
-  new (type: T): GraphQLNonNull<T>;
-}
-
-// eslint-disable-next-line @typescript-eslint/no-redeclare
-export const GraphQLNonNull: _GraphQLNonNull<GraphQLNullableType>;
-
 export type GraphQLWrappingType = GraphQLList<any> | GraphQLNonNull<any>;
 
 export function isWrappingType(type: any): type is GraphQLWrappingType;
diff --git a/src/type/definition.js b/src/type/definition.js
index e3a8c5c722..6ffaa4f906 100644
--- a/src/type/definition.js
+++ b/src/type/definition.js
@@ -345,39 +345,31 @@ export function assertAbstractType(type: mixed): GraphQLAbstractType {
  *     })
  *
  */
-// FIXME: workaround to fix issue with Babel parser
-/* ::
-declare class GraphQLList<+T: GraphQLType> {
+export class GraphQLList<+T: GraphQLType> {
   +ofType: T;
-  static <T>(ofType: T): GraphQLList<T>;
-  // Note: constructors cannot be used for covariant types. Drop the "new".
-  constructor(ofType: GraphQLType): void;
-}
-*/
 
-export function GraphQLList(ofType) {
-  // istanbul ignore else (to be removed in v16.0.0)
-  if (this instanceof GraphQLList) {
-    this.ofType = assertType(ofType);
-  } else {
-    return new GraphQLList(ofType);
+  constructor(ofType: T) {
+    devAssert(
+      isType(ofType),
+      `Expected ${inspect(ofType)} to be a GraphQL type.`,
+    );
+
+    this.ofType = ofType;
   }
-}
 
-// Need to cast through any to alter the prototype.
-(GraphQLList.prototype: any).toString = function toString() {
-  return '[' + String(this.ofType) + ']';
-};
+  toString(): string {
+    return '[' + String(this.ofType) + ']';
+  }
 
-(GraphQLList.prototype: any).toJSON = function toJSON() {
-  return this.toString();
-};
+  toJSON(): string {
+    return this.toString();
+  }
 
-Object.defineProperty(GraphQLList.prototype, SYMBOL_TO_STRING_TAG, {
-  get() {
+  // $FlowFixMe[unsupported-syntax] Flow doesn't support computed properties yet
+  get [SYMBOL_TO_STRING_TAG]() {
     return 'GraphQLList';
-  },
-});
+  }
+}
 
 // Print a simplified form when appearing in `inspect` and `util.inspect`.
 defineInspect(GraphQLList);
@@ -402,39 +394,31 @@ defineInspect(GraphQLList);
  *
  * Note: the enforcement of non-nullability occurs within the executor.
  */
-// FIXME: workaround to fix issue with Babel parser
-/* ::
-declare class GraphQLNonNull<+T: GraphQLNullableType> {
+export class GraphQLNonNull<+T: GraphQLNullableType> {
   +ofType: T;
-  static <T>(ofType: T): GraphQLNonNull<T>;
-  // Note: constructors cannot be used for covariant types. Drop the "new".
-  constructor(ofType: GraphQLType): void;
-}
-*/
 
-export function GraphQLNonNull(ofType) {
-  // istanbul ignore else (to be removed in v16.0.0)
-  if (this instanceof GraphQLNonNull) {
-    this.ofType = assertNullableType(ofType);
-  } else {
-    return new GraphQLNonNull(ofType);
+  constructor(ofType: T) {
+    devAssert(
+      isNullableType(ofType),
+      `Expected ${inspect(ofType)} to be a GraphQL nullable type.`,
+    );
+
+    this.ofType = ofType;
   }
-}
 
-// Need to cast through any to alter the prototype.
-(GraphQLNonNull.prototype: any).toString = function toString() {
-  return String(this.ofType) + '!';
-};
+  toString(): string {
+    return String(this.ofType) + '!';
+  }
 
-(GraphQLNonNull.prototype: any).toJSON = function toJSON() {
-  return this.toString();
-};
+  toJSON(): string {
+    return this.toString();
+  }
 
-Object.defineProperty(GraphQLNonNull.prototype, SYMBOL_TO_STRING_TAG, {
-  get() {
+  // $FlowFixMe[unsupported-syntax] Flow doesn't support computed properties yet
+  get [SYMBOL_TO_STRING_TAG]() {
     return 'GraphQLNonNull';
-  },
-});
+  }
+}
 
 // Print a simplified form when appearing in `inspect` and `util.inspect`.
 defineInspect(GraphQLNonNull);
diff --git a/src/utilities/extendSchema.js b/src/utilities/extendSchema.js
index 24e763a745..5bdf49e03b 100644
--- a/src/utilities/extendSchema.js
+++ b/src/utilities/extendSchema.js
@@ -432,6 +432,7 @@ export function extendSchemaImpl(
       return new GraphQLList(getWrappedType(node.type));
     }
     if (node.kind === Kind.NON_NULL_TYPE) {
+      // $FlowFixMe[incompatible-call]
       return new GraphQLNonNull(getWrappedType(node.type));
     }
     return getNamedType(node);